Study On Fatigue Of Air-Engine Blades By Damage Mechanics And Influence Of Foreign Object Damage

With the requirements of modern transport and national defense, civil and military aircraft are rapidly developing and the performance, reliability and safety of gas turbine engine will become higher and higher. The increase of thrust weight ratio, as a key measure of engine performance, is obtained from the applications of new materials, structures, advanced manufacture method etc, and so the properties of material are improved and working stress margins are reduced, at same time, the operation condition of engine components/parts becomes much more serious or deteriorative and its stress level is higher, thus structural integrity, safety and reliability become a key restraining the fast development of engine.Blade is major parts of aircraft engine. The blades, especially rotor blades, are complicated in structure and loading. thin and long in size, work in high and variable rotation speed, and are subject to impact of air and gas flow that can result in vibration. Compressor blades located in front of engine may also be damaged by the impact of sand, stone and other foreign objects ingested into engine. Low cycle stress caused by the variable rotating speed and the high cycle stress from high frequency vibration excited by unsteady flow make blade damaged increasingly, until fatigue failure occurs. The blade fracture may break compressor or whole engine, furthermore lead to aircraft accident.It is the one of the major objectives and important part of air engine structural integrity and reliability to study the law of high and low cycle fatigue, to refine fatigue analysis method and to improve the level of the fatigue damage assessment and life prediction. In this paper, based on continuum damage mechanics (CDM) and fatigue damage test, fatigue damage of rotor blades under low and high cycle loading was systematically analyzed and studied, a FE program system for whole CDM-analysis for fatigue damage of compressor rotor blades was developed, which provides a new approach to blade fatigue analysis. Furthermore, foreign object damage (FOD) that seriously reduces blade life were investigated through test and numerical modeling. The main researches are as follows:(1) Investigation of damage mechanics model and analytical method for blade low cycle fatigue.Using the existing tests results, the rationality and advantages of damage mechanics approach to fatigue analysis were discussed from the short crack mechanism of fatigue. Based on the present strain-controlled fatigue test data, the traditional low cycle fatigue damage model was refined which is good agreement with the test result in all fatigue damage process. A simplified method for damage evolution analysis and life prediction of low cycle fatigue has been developed according to low fatigue damage evolution of titanium alloy Ti-6Al-4V, as well as a convenient inverse analysis to determine the damage parameters of materials.(2) Investigation of damage mechanics model and analytical method for blade high cycle fatigue.Based on high cycle fatigue damage mechanism, the damage evolution equation was derived. The proposed method was extended and applied to high cycle fatigue caused by vibration stress.(3) The strain-controlled fatigue damage test in four strain amplitudes was completed for the standard specimens of the material TC4 used for compressor blade. Characteristics of strain softening, damage growth and fatigue damage process, variation of loop and limit of damage variable were obtained, and the test data and essential parameter is provided for the study of damage model and analysis method, as well as numerical calculation.(4) FE software system (by Fortran, with 10000 sentences) has been developed for low and high cycle fatigue analysis by damage mechanics.In order to provide research tools and platform for the study of this paper and application, FE analysis software has been developed for blade fatigue damage mechanics with 3D 20 nodes hexahedron isoperimetric element. The system have the function of elasto-plastic analysis, vibration analysis, low cycle and high cycle fatigue damage evolution analysis, life prediction and better pre-processing and past-processing. The programs has been fully validated.(5) Fatigue damage analysis for typical blade.With the above model, method, and programs, A series of analysis was carried out for typical compressor blade, including cycle elasto-plastic stress, dynamic frequency and static frequency, vibration stress, fatigue damage field and fatigue life in given load cycle, and result were discussed. The result show that the refined damage model, the proposed simplified methods and the software are available.(6) Investigation of foreign object damage(FOD) on blade leading edge and its influence on high cycle fatigueThe connection of the indent depth and impact energy were obtained by impacting on specimens simulating blade leading edge using self-made lift hammer in the free falling body. A combined the stress-controlled fatigue test and numerical analysis was done with the damaged specimens which give the residual life vs stress amplitudes.